In a large marjority of rotary drilling machines used in applications such as water well, shallow oil and gas, blast hole and exploration drilling, the top and bottom of the derrick or mast or tower is used to support the loading of actual drilling and extraction operations.
Loading is induced at the tower or mast top and bottom by transmitting forces generated by a hydraulic cylinder through a cable or chain arrangement over sheaves or sprockets in the top and bottom to direct the force to raise or lower the drilling head. This loading is transferred to the mast structure via the crown sheaves or sprockets at the top and bottom sheaves or sprockets and requires that the mast be designed to accept loads two or three times the actual rotary head loads.